What are the JPA entity states, and how do persist and merge differ?

Connects transient, managed, detached, and removed states to persist, merge, dirty checking, and safe update workflows.

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TL;DR

A new or transient entity is not associated with a persistence context. A managed entity is tracked and participates in dirty checking. A detached entity has identity but is no longer tracked, and a removed entity is scheduled for deletion. persist makes a new entity managed. merge copies state from a detached or new object into a managed instance and returns that managed instance; the object passed to merge does not itself become managed.

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Entity lifecycle states, persist versus merge, detached data, managed return values, and update correctness.

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Most surprising JPA behavior becomes easier to explain once the entity's state is known.
A new or transient entity is not associated with a persistence context. A managed entity is tracked and participates in dirty checking. A detached entity has identity but is no longer tracked, and a removed entity is scheduled for deletion. persist makes a new entity managed. merge copies state from a detached or new object into a managed instance and returns that managed instance; the object passed to merge does not itself become managed.
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That return-value rule matters: continuing to modify the detached argument after may not persist those changes. Blindly merging a request-shaped entity can also overwrite fields the request never owned, depending on mapping and loaded state. In a normal service update I load the managed entity inside the transaction, apply allowed changes, and let dirty checking write them. This preserves authorization, invariants, and optimistic-version checks more clearly than reconstructing a partial entity.

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Saying merge attaches the object passed to it.
Calling persist for an arbitrary detached entity.
Merging a request body directly into the database model.

What are the JPA entity states, and how do persist and merge differ?

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